From Imps to Demons
Completing the Bestiary is quite easy, as long as you remember a few things. Not all enemies can be encountered during the natural course of the story if you avoid all Demon Scrolls. You also need to defeat the Kusa 5 for the "Who Let the Dogs Out" achievement.
Doing so will add the following entries that you'd otherwise be missing:
- Canine Warrior Rei
- Canine Warrior Shin
- Canine Warrior Chi
- Canine Warrior Ko
While you encounter all of them while progressing in the Kusa Village's storyline, their bestiary entry won't appear until you fight and defeat them.
Other missable enemies are:
- Fire Doom Mirror, that you can find in the Emperor's room, in one of the scrolls just below the Platform Spiders.
- Clay Flyer. It can be found in Shinshu Field 100 years in the past, but you'll have to sift through the Demon Scrolls in the area until you find it.
- Umbrella Namahage in Kamui. Either fight in the Devil Gate trial or just look for it in the scrolls.
Fighting the Bandit Spiders (see "Out of the Gate Swinging") is also required.